When fastening profiled metal sheeting, crest fixing means securing the sheet through the upper ridges (crests), while valley fixing secures it through the lower channels (valleys or pans). The primary difference is how effectively each method prevents water leakage.
Crest fixing
- What it is: Fastening through the high points (crests) of the metal sheets.
- Best for: Roofing applications, especially corrugated roofing.
Advantages:
- More weathertight as the fastener is less exposed to water and debris.
- Eliminates the risk of a screw head acting as a dam in a valley, which can cause leaks if the washer fails.
Disadvantages:
- Can create an indent around the screw, causing water to pool around the screw
In general crest fixing is the standard and most watertight method for installing metal roofing sheets.
- Mechanism: Fasteners, with a waterproof washer, are driven through the highest part of the metal profile into the support structure beneath.
- Water-tightness: By fixing on the crest, the fastener is elevated above the primary water channel. Even if the washer fails, the water will likely flow down the valley and away from the fastener hole, minimizing the risk of leaks.
- Application: Used for roofing installations where maximum water-tightness is critical.
Valley fixing
What it is: Fastening through the low points (valleys or pans) of the metal sheets.
Best for: Walling applications where a clean aesthetic is desired and the fasteners are less visible.
Advantages:
- Provides a cleaner, less visible fastener line, as the fasteners are hidden in the valleys.
- The sheet rests flat against the support structure, which can prevent deformation.
Disadvantages:
- More prone to leaks, as the fastener is in a location where water and debris can collect. Regular roof maintenance however eliminates this risk.
This method is generally not recommended for roofing due to a high risk of possible leaks but is suitable for wall cladding.
- Mechanism: Fasteners are driven through the lowest part of the metal profile (the valley or pan).
- Water-tightness: Because the fastener is in the path of draining water, any failure in the washer or sealant will cause water to pool around the fastener and eventually leak into the building.
- Application: Reserved for vertical wall cladding, where water simply runs down the surface and does not pool.
Comparison table
Feature | Crest Fixing | Valley Fixing |
Location | The upper ridges or crests of the metal profile. | The lower channels or valleys (pans) of the metal profile. |
Risk of Leaks | Low, as the fastener is above the water line. | High, as water flows directly over the fastener. |
Best For | Roofing applications to ensure maximum water-tightness. | Vertical wall cladding applications. |
Fastener | Uses a self-drilling fastener with a waterproof EPDM washer. | May use a full-threaded screw, but leaks are still a major risk on roofs. |
Which one to choose?
You decide. 6 of the one, half a dozen of the other… My personal choice is Valley Fixing because if you do regular roof maintenance, no debris will accumulate around the fastener thus eliminating the only disadvantage of this type of fixing. The big advantage of Valley Fixing is that, even if you (or your labourer) do overtighten the fastener, you cannot create an indent causing water to pool around the screw and leak. Second benefit is that you can use #12x50mm (instead of #12×65/85mm) for fixing into Timber or #12x38mm (instead of #12×65/85mm) for fixing onto Metal – a huge cost saving factor.
But that is only me.
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